Precambrian Potassic Ultramafic Rocks: South Greenland
Small hypabyssal intrusives of biotite pyroxenite and biotite peridotite are described from part of theGardar alkaline province. The intrusives, composed essentially of diopside, olivine, phlogopite and Fe-Ti oxides (and numerous accessory minerals) are inferred to have crystallized from silica-unde...
| Published in: | Journal of Petrology |
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| Main Authors: | , |
| Format: | Text |
| Language: | English |
| Published: |
Oxford University Press
1973
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| Subjects: | |
| Online Access: | http://petrology.oxfordjournals.org/cgi/content/short/14/3/509 https://doi.org/10.1093/petrology/14.3.509 |
| Summary: | Small hypabyssal intrusives of biotite pyroxenite and biotite peridotite are described from part of theGardar alkaline province. The intrusives, composed essentially of diopside, olivine, phlogopite and Fe-Ti oxides (and numerous accessory minerals) are inferred to have crystallized from silica-undersaturated ultramafic magmas of similar bulk composition. Crystallization occurred at an estimated depth of 3–4 km. Despite relatively rapid consolidation (probably attendant on a partial devolatilization) producing typically fine-grained and sometimes porphyritic rocks, small-scale differentiation nevertheless occurred. The differentiates, as veins, pegmatoidal patches and roofing-facies tend to be olivine-poor phlogopite+ferriandiopside+ Fe-Ti oxide rocks, in which perovskite, andradite, apatite and sphene are significant components. Pyroxene compositions in the ultramafites are consistent with the thesis that the rocks formed from soda-deficient magmas in strongly oxidizing conditions. Relative potassium enrichment (average K 2 O: Na 2 O wt. % = 1.6) is expressed modally in the ubiquitous presence of phlogopite and normatively by an average of 6.9% or and/or Ic . Despite compositional affinity with certain olivine melilitites, melilite is absent, a fact that may be attributable to the comparatively high hydrostatic pressure ( c . 1 kb) pertaining during crystallization. Petrographic and geochemical evidence suggests that the ultramafic magmas were residua after fractionation of forsteritic olivine from more magnesian magmas of kimberlite type. It is proposed that the latter may in turn have been residua from eclogite fractionation of Fe-Ti rich primary magmas in the upper mantle. |
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